Recent years, in large capacity trunk line optical communication systems having a capacity of over 100 Giga bit per second (Gbps), a digital coherent method has been used. Such trunk line optical communication systems are sometimes configured as elastic network systems in which offloading of communication traffic to transmission pathways having a light amount of communication traffic and flexible transmission line switching, such as bypassing of a transmission line where a failure has occurred, can be performed. In an optical communication system having such a configuration, it is preferable to continue stable communication even when transmission conditions have changed.
In large capacity trunk line optical communication systems, communication based on a multi-level modulation scheme, such as a quadrature phase shift keying (QPSK) method and 16-quadrature amplitude modulation (QAM), has been put to practical use. Aiming at further increasing capacity, development of higher order multi-level modulation scheme, such as 32-QAM and 64-QAM, has been under way.
In parallel with such capacity increase through multi-level schemes, research and development of transmission methods improving frequency usage efficiency by narrowing signal bands and wavelength-division-multiplexing the signals (wavelength division multiplexer (WDM)) has been actively conducted. Typical examples of such transmission methods include a Nyquist transmission method that enables further band narrowing of signal spectra than transmission based on a non-return zero (NRZ) scheme, which has been widely used so far. Advanced signal bandwidth narrowing technologies, such as a super-Nyquist transmission method in which signal bandwidth is made narrower than a signal baud rate, have also been researched. As described above, an approach to narrow channel spacing at the time of wavelength division multiplex transmission by narrowing signal bandwidth and thereby increase transmission capacity per optical fiber is regarded as important as well as the multi-level scheme.
In particular, for an optical transmission system having a capacity of over 1 Tera bit per second (Tbps), a subcarrier multiplexing method in which a plurality of subcarriers are wavelength-division-multiplexed and 1 Tbps transmission is thereby achieved is effective in consideration of feasibility. Since frequency usage efficiency increases as subcarriers become more densely spaced, a technology for performing transmission through narrowing spacings at which subcarriers are multiplexed becomes important. Thus, development of technologies for performing wavelength-division-multiplexed transmission through narrowing subcarrier spacings has been actively conducted. As the technology for performing wavelength-division-multiplexed transmission through narrowing subcarrier spacings, for example, a technology described in NPL 1 has been disclosed.
An optical transmission system in NPL 1 is a communication system that performs transmission of polarization-multiplexed multi-level optical signals in accordance with the digital coherent method. In the optical transmission system in NPL 1, spectral shaping processing for narrowing the band of main signals is applied. NPL 1 asserts that performing band narrowing, while enabling inter-channel interference to be suppressed, enables wavelength-division-multiplexing spacings to be narrowed.